This invention relates to a method of making ohmic contacts to n-type diamond and injecting contacts to p-type diamond.
n-Type diamond can be prepared by suitable implantation-annealing sequences of the required donor atoms, for example phosphorus. This causes a massive movement in the position of the Fermi level towards the conduction band, which, in turn, makes it difficult to effect ohmic contacts to such a layer.
In the case of p-type diamond containing boron acceptors, many different methods have been devised to prepare ohmic contacts. Contacts with low resistance can be prepared by first overdoping a thin layer at the surface of the contact with boron acceptors. Such a layer allows the current or charge carriers to tunnel into the diamond thus eliminating any unwanted rectifying behaviour. A popular way to achieve the latter is to implant the surface with boron ions (typically at energies varying from 30 to 60 keV) and to a very high dose (typically larger than 1.times.10.sup.16 ions cm.sup.-2) while maintaining the diamond at temperatures between room temperature and approximately 300.degree. C. At these high ion doses, the implanted surface layer becomes amorphous. After a subsequent anneal, typically at temperatures ranging from 600 to 1400.degree. C., the layer becomes graphitic and can be etched off by using a suitable oxidising acid solution. After etching the remaining surface then contains a very high density of boron acceptors, which facilitates the preparation of ohmic contacts.
This method does not work for larger atoms like phosphorus. After following the same procedure, the etched surface does not contain a large density of phosphorus atoms. Although it is possible to dope the diamond n-type, it is more difficult to prepare an overdoped surface. It is known that highly damaged diamond, prepared, for example, by implanting carbon ions to a high does under similar conditions as the boron-ions mentioned above can act like n-type diamond by creating a p-n junction when prepared on a p-type semiconducting diamond. One would thus expect that a similar treatment of n-type diamond may lead to ohmic contacts. However, this was found not to be the case.